In analytical chemistry, preconcentrators have been used for many years to collect molecules that are present in low concentrations. Analytical instruments may not be able to detect molecules in such low concentrations. Preconcentrators accumulate and concentrate one or more chemical species of interest over time, so that the analytical instruments can detect the molecule. Thus, preconcentrators increase the sensitivity of analytical techniques such as, e.g., gas chromatography, mass spectrometry, and ion mobility spectrometry (IMS).
Preconcentrators are particularly useful to aid in the detection of trace compounds such as drugs, explosives, and other toxic agents. As these compounds are typically found in the field, battery-powered portable detectors have been developed.
The key feature of a preconcentrator is the ability to adsorb an analyte and then release it at a specific temperature. To adsorb the analyte, special materials called adsorbent resins have been developed. Adsorbent resins are typically high surface area powders and the nature of the analyte determines the choice of resin.
Existing preconcentrators usually consist of an adsorbent ‘slug’ inside a tube. The sample passes through the tube and analytes adsorb onto the slug. When enough analyte has accumulated, the slug is heated to release a concentrated ‘plume’ of analyte into the detector for techniques such as e.g. IMS. These preconcentrators have a low surface area to volume ratio, requiring a long time to accumulate a sufficient quantity of analyte. Furthermore, due to a pressure drop across the preconcentrator, inline use with existing detectors may require changing the internal air handling. Such changes can be difficult, expensive and even preclude retrofitting of preconcentrators to an existing device. The slug is also large requiring a fair amount of time and energy to release the analyte. This energy consumption poses a particular problem when preconcentrators are used in portable detection systems as it lowers the battery life.
For portable systems, micro-machined preconcentrators have been designed. Typical inline micro-machined preconcentrators consist of a thin film serpentine structure with an adsorbent coating on top. The structure can have thickness in the order of microns and consequently is quite fragile. The heating element is external to the device, limiting thermal efficiency. A break in the structure, which also serves as the heating track, will ordinarily cause complete failure.
The surface area of such concentrators is essentially the surface area of the top of the structure, as the thickness is negligible. As a result, such devices have a relative low surface area to which the analyte adsorbs. Furthermore, because of their low surface area it takes a longer time to preconcentrate the analyte. Once sufficient analyte has accumulated, current is passed through the structure and causes desorption. Since the heating of the preconcentrator is often not uniform, additional time and energy are required to desorb the analyte. Furthermore, due to the non-uniform heating, it is difficult to accurately control desorption of the analyte.
Micro-machined preconcentrators may be mounted inline to the detector or externally. In an external preconcentrator, the preconcentrator located inside a chamber and the analyte enters through an inlet port and leaves through an outlet port. Such preconcentrators are disadvantageous in that they add complexity to the apparatus and thus hinder further miniaturization.
U.S. Pat. No. 6,239,428 to Kant discloses systems and methods of ion mobility spectrometry. The system may contain a preconcentrator whose temperature is modulated between two temperatures. The preconcentrator has permeable organic membranes or thin metal foils. Consequently, the preconcentrator has low surface area and is quite fragile.
U.S. Pat. No. 6,171,378 to Manginelli et al. is illustrative of a micro-machined external preconcentrator. The preconcentrator contains a substrate with a suspended membrane, which serves to support two resistive heating elements on top of which an adsorbent coating is deposited. Again, this preconcentrator does not maximize the surface area.
During the manufacture of a micro-machined preconcentrator, preconcentration material is placed on the device. One way to deposit the preconcentration material is to use ink jet deposition. This process employs about 70,000 individual drops and is slow and serial. Ink jet deposition lacks resolution to create ultra-small geometries and when complex features have to be printed, it can be prohibitively expensive.
There remains a need for a preconcentrator that does not create a large pressure drop, requires little energy to heat, can be micro-machined and improves the preconcentration abilities. There also remains a need for a cheap, efficient, and accurate method of manufacture of a micro-machined preconcentrator.